# HEAT TRANSFER - 2

Considering unit cross -section area of ice , if alayer of thickness dx given in time dt, The heat given by thin layer.

= mass x Latent heat

= 1.dx..L.

= density of ice

L = Latent heat of fusion of ice.

The quantity of heat is conducted upward through the ice layer in time dt

dx..L. =

Time taken

__ Dumb Question__:

Q1: Pieces of iron and glass are heated to same temperature. which one will be more hot ? why ?

A: Pieces of iron wil be hotter. Because of higher thermal conductivity.

Q: Cooking utensils are provided wth wooden handes why ?

A: Wood is pure conductor of heat. So hot utensils can be easily handled.

Q: In winter, metallic handles of wooden door appear colder. Why ?

A: Because they are good conductor of heat.

Q: Why is ice packed in gunny bags or sawdust.

A: Air trapped in saw dust prevent transfer of heat from the surroundings, to the ice.

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**Illustration**Q: A cubical ice box of thermocole has each side = 30cm and a thickness of 5cm. 4kg of ice is put in the box. If outside temp is 45

^{0}c and the coeff. of thermal conductivity = 0.01 J s

^{-1}m

^{-1}

^{0}c

^{-1}Calculate the mass of ice left after 6 hrs. Take latent heat of fusion of ice = 335 x 10

^{3}J/kg.

Solution: l = 0.3 m

Thickness = 0.05 m

Total surface area = 6l

^{2}

= 0.54 m

^{2}

T = 45

^{0}c

T = 6 hrs

Q = mL = KA

m = KA

= 0.313 kg

wt of ice left = 3.687 kg

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**Radation**__:__

**Absorptive Power a**Absorptive power of a body is defined as the fraction of the incident radiation that is absorbed by the body.

Absorptive Power a =

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**Emmisive Power E**The emmisive power denotes the energy radiated per unit time per unit area of the surface.

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**Emmisitivity E**The emmisitivity of a surface is the ratio of the Emmisive power of the surface to the emmisive power of the black body at that temp.

E =

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**Black Body**A perfectly black body is the one which absorbs completely all the radiations of whatever wavelength incident on it.

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**Kirchoff’s Law**It states that the ratio of the Emmisive power to the Absorptive power for radiation of a given wavelength is the same for all bodies at the same temperature.

[constant]

[Secondary Information]

Everything is continuously emitting radient energy in all direction at a rate depending only on the nature of the surface and its temperature and also it is absorbing radient energy from all surrounding bodies at arate depending on its surface and temperature of the surrounding bodies